Polyester monofilaments have traditionally been used in the paper making industry. Such monofilaments are frequently woven into support belts or fabrics for transporting and dewatering paper sheets produced by paper-making machines. While in use, these fabrics are subjected to demanding conditions that chemically, physically, and mechanically degrade the polyester monofilaments from which the fabrics are made. Specifically, these fabrics are typically subjected to thermal, hydrolytic and abrasive conditions.
Traditionally these fabrics have been manufactured from monofilaments prepared by melt extruding standard polyester resins such as polyethylene terephthalate (PET). This polyester is well-known in the art and has long been used in the production of polyester monofilaments that are suitable for use in the manufacture of paper machine fabrics. PET has a known melting point of less than 260.degree. C. and can be readily adapted for monofilament use. However, while PET has relatively good dry heat (thermal) stability, it has only moderate hydrolytic stability as compared to polyester resins having higher melt temperatures. Furthermore, PET monofilaments have only moderate toughness to abrasion since such monofilaments generally may require replacement within about 30 to 60 days on wear prone forming positions.
With regard to hydrolytic degradation, attempts have been made to improve the hydrolytic stability of PET. For example, Barnewall, U.S. Pat. No. 3,975,329, indicates that the hydrolytic as well as the thermal stability of PET can be improved by melt extruding this standard polyester resin in the presence of a significant amount of a carbodiimide. Specifically, the patent indicates that the amount of carbodiimide used should be equal to the concentration of carboxyl groups in the original resin plus the concentration of carboxyl groups generated when the original resin is extruded in the absence of carbodiimide.
With regard to toughness and abrasion resistance, nylon monofilaments have often been used in combination with polyester monofilaments on high wear positions. The use of nylon, however, may cause some problems in this type of usage due to its high moisture absorption. It has also been known in the art to blend certain fluoropolymers with various thermoplastic resins to achieve a number of desired results. For example, Busse et al. U.S. Pat. No. 3,005,795 teaches the blending of polytetrafluoroethylene (hereinafter PTFE) in powder form to various thermoplastic polymers such as methacrylate polymers, styrene polymers, and polycarbonates. Schmitt et al. U.S. Pat. No. 3,294,871 teaches the blending of PTFE in latex form to various thermoplastic polymers including those mentioned hereinabove. However, in both of these patents, the blends included finely divided microfibrous particles of PTFE which are not suitable for producing polyester monofilaments, as discussed hereinbelow.
At least two patents have blended PTFE with a polyester resin. Notably, Lucas U.S. Pat. No. 3,723,373 teaches the addition of a PTFE emulsion to polyethylene terephthalate (PET) to achieve a material which has greater elongation and improved impact strength. The PTFE emulsion is merely PTFE in the form of a latex dispersion or emulsion with water, mineral oil, benzene or the like. Accordingly, the PTFE emulsion also includes particles of about 0.1 micron to about 0.5 microns in size which comprise about 30 to 80 percent of the emulsion. The PTFE emulsion forms about 0.1 to 2.0 percent by weight of the blend, based upon the weight of the PET. Furthermore, Lucas indicates that this material can be extruded into sheet or stock shapes at a temperature of around 260.degree. C.
Similar to Lucas, Smith U.S. Pat. No. 4,191,678 relates to a fire retardant polymer blend comprising an aqueous colloidal dispersion of PTFE and a polyester resin. Again, however, the PTFE in the dispersion has an average particle size of about 0.2 microns. Smith also indicates that the blend may be subsequently extruded at about 240.degree. C.
The extrusion temperatures of these blends have been noted because it is well known that the melt temperature of PTFE is between about 335.degree. C. and about 343.degree. C. (635.degree.-650.degree. F.), and therefore, when PTFE and the polyester resin are extruded under standard operating conditions at temperatures below 320.degree. C. (608.degree. F.), such as taught by at least one of the above-identified patents, it is clear that the PTFE in the blend must be in the form of solid particles and not in the form of a liquid melt. Importantly, such blends having PTFE in particle form have been found to produce polyester monofilaments that are insufficient for use in paper maker fabrics. The polyester monofilaments are very difficult to extrude because the particles can easily clog or otherwise damage the extrusion equipment that is geared toward producing monofilaments from melted blends. Additionally, when polyester monofilaments are produced from these blends, they have been found to be very rough and not suitable for use in paper maker fabrics. Furthermore, and possibly even more importantly, the PTFE retains its useful properties only up to about 287.degree. C. (550.degree. F.). Accordingly, by melting the PTFE at higher temperatures, all advantages gained by the inclusion of PTFE in these blends would be lost.
Thus, a need exists for a fabric polyester monofilament that is hydrolytically stable and that demonstrates an improved resistance to abrasion and contamination. Attempts have been made to improve the abrasion resistance of monofilaments produced from PET while also improving the hydrolytic stability of the monofilament. For example, Masuda et al., U.S. Pat. No. 5,378,537, teaches a PET monofilament stabilized by the addition of an unaltered carbodiimide compound in the range of from 0.005 to 1.5 percent by weight and a fluorine type polymer in an amount in the range of from 0.01 to 30 percent by weight. The resulting polyester monofilament provides a superior resistance to hydrolysis and proof against staining, compared with the conventional countertype. Despite these improvements, however, Masuda et al. teaches that the physical properties of the monofilament deteriorate when the concentration of the carbodiimide exceeds 1.5 percent by weight.
Therefore, a need still exists, as a result of the deleterious conditions that paper machine fabrics are subjected to during the paper making process, to improve the hydrolytic stability of PET monofilaments, and fabrics made therefrom, while not dissipating the physical properties of the polyester monofilament where amounts larger than 1.5 percent by weight hydrolytic stabilizer are used. Moreover, a further need still exists to improve the abrasion resistance of PET monofilaments, and fabrics made therefrom, in conjunction with improving the hydrolytic stability of such monofilaments and related fabrics while not dissipating the physical properties of the monofilaments or fabrics.